Such a centrifugal separator is disclosed in SU-1194504 which discloses a centrifugal separator with a centrifuge rotor fed from above through a central channel. The centrifuge rotor, which is driven by means of an electric motor. The electric motor comprises a stator, is fixedly mounted to a stationary casing, and a rotor which is fixedly mounted to the rotating casing of the centrifuge rotor. The electric motor is thus located axially at the level of the centrifuge rotor.
GB-A-2 277 700 discloses a centrifugal separator, where the centrifuge rotor is mounted directly to the spindle which in turn is directly connected to an electric drive motor which thus is located outside the centrifuge rotor. With such an arrangement, one of the ends of the centrifuge rotor is accessible for supply and/or discharge of material to and from the inner separation space of the centrifuge rotor. In order to enable supply and discharge through both ends of the centrifuge rotor, it is known to mount the centrifuge rotor on a so-called hollow spindle, SE-B-348121. With such a centrifuge rotor, the driving may take place by means of a motor which is provided beside the centrifuge rotor and the spindle. The power transmission from the drive motor to the spindle is performed by means of, for instance, drive belts or a screw gear.
The electric motors which available today may be controlled in a and operated at high number, of revolutions per minute. It is therefore desirable to be able to operate the centrifuge rotor by direct drive.
DE-A-10300976 discloses a centrifuge comprising a stationary shaft on which a stator is mounted. A rotor having rotor blades is journalled on the shaft. The drive motor is centrally provided in the centre of the centrifuge inside the rotor. In another embodiment, a centrifuge is described that has a rotor supplied with a medium via a stationary central channel. The driving is performed by means of electromagnetic coils provided on a stationary casing outside the rotor. The rotor comprises a disc with alternating magnetisable areas.
U.S. Pat. No. 6,709,477 discloses a centrifugal separator having a hollow spindle. The centrifuge is intended for turbine drive by means of an inlet pipe and turbine rotor.
US2002/0016245 discloses a centrifugal separator having a hollow spindle and an inlet from beneath and an outlet from above. The driving is not described more closely.
GB-A-409,196 discloses a centrifugal separator having a hollow spindle for feeding from beneath. The spindle is journalled in two bearings. The driving is not described more closely.
WO 98/45050 discloses a device for dynamic classification of carbon powder. The device comprises a rotating spindle which carries a rotor member consisting of axially and radially directed wings. The spindle is hollow and extends through a drive motor having a stator and a rotor. The spindle is fixedly attached to the rotor of the drive motor.
USB-2001-0012814 discloses a centrifugal filter for filtering partials from a liquid. One embodiment refers to a rotor in the form of a filter element with a rotating hollow shaft and a casing. On or radially outside the shaft, filter means are provided. This filter means do not constitute separation discs. The purpose of these filter means is to filtrate liquid and catch particles in the filter means. On the rotor, magnets are mounted. By means of the rotor magnets, an electric motor is created with a stator not disclosed. Liquid to be filtered may be supplied via an inlet and upwardly through a stationary shaft and into the rotating hollow shaft of the rotor. An outlet for the filtered liquid is provided at a large distance from the centre axis.